Investment casting process and apparatus to facilitate superior grain structure in a DS turbine bucket with shroud

ABSTRACT

An investment casting process that enables directionally solidified tip shrouded turbine blades or buckets to have a continuous grain structure that extends through the tip shroud in addition to increasing the quantity of grains in the root of the part.

BACKGROUND OF THE INVENTION

A gas turbine is typically comprised of a compressor section that produces compressed air. Fuel is mixed with a portion of the compressed air and burned in one or more combustors, thereby producing hot compressed gas. The hot compressed gas is expanded in a turbine section to produce rotating shaft power. The turbine section is typically comprised of a plurality of alternating rows of stationary vanes (nozzles) and rotating blades (buckets). Each of the rotating blades has an airfoil portion and a root portion by which it is affixed to a rotor.

On many rotating airfoils, integral tip shrouds are used on the radially outer end of the blade to create an outer surface of the passage through which the hot gases must pass. Having the shroud as a part of the airfoil results in an increase in performance for the engine. As such, it is desirable for the entire outer surface to be covered by the tip shrouds. However, integral shrouds on rotating airfoils are highly stressed parts due to the mechanical forces applied via the rotational speed. The high temperature environment coupled with the high stresses makes it a challenge to design a shroud that will effectively perform over the entire useful life of the remainder of the blade. One weak area of the shroud is the fillet between the airfoil and tip shroud. One possibility for resolving this challenge is to reduce the stress applied to the tip shroud fillet. One common method is to scallop or remove a portion of the overhanging shroud, thus reducing the load applied. However, physically removing tip shroud coverage results in a detriment to engine performance.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to a blade for a turbine, e.g. aircraft engine, gas turbine, steam turbine, etc. More specifically, the present invention relates to an investment casting process that enables directionally solidified tip shrouded turbine blades or buckets to have a continuous grain structure that extends through the tip shroud in addition to increasing the quantity of grains in the root of the part. The invention may be readily applied to land-based turbine buckets or aircraft engine turbine blades.

Thus, the invention may be embodied in an investment casting process for forming a directionally solidified blade comprising: providing a blade mold, said mold being oriented so that a portion of a mold cavity thereof for forming a base of said blade is at a base thereof and a portion of the mold cavity for forming a tip of the blade is at a vertically upper end thereof; providing a heat removal feature to extend below said mold; flowing molten metal into said mold cavity of said mold; and allowing said blade to solidify from the base upwardly.

The invention may also be embodied in an investment casting assembly for molding a directionally solidified blade comprising: a blade mold, said mold being oriented so that a portion of a mold cavity thereof for forming a base of said blade is at a base thereof and a portion of the mold cavity for forming a tip of the blade is at a vertically upper end thereof; a heat removal feature disposed to extend below said mold; and a plumbing system for flowing molten metal into said mold cavity of said mold, whereby the molded blade will solidify from the base upwardly.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic perspective view of a turbine blade with tip shroud;

FIG. 2 is a schematic plan view of conventional tip shrouds, illustrating shroud scalloping;

FIG. 3 is a schematic illustration of a typical turbine blade casting arrangement; and

FIG. 4 is a schematic illustration of a turbine blade casting arrangement according to the process of an example embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A typical blade with cooling passages exiting at the blade tip to flow over the tip shroud is schematically illustrated in FIG. 1. As schematically illustrated therein, each turbine blade 10 is comprised of an airfoil portion 12 and a root portion 14. The airfoil portion has a leading edge and a trailing edge. A generally concave pressure surface and a generally convex suction surface extend between the leading and trailing edges on opposing sides of the airfoil. In the illustrated example, the blade root 14 is comprised of a shank 16 and a dovetail 18 to engage a corresponding dovetail groove on the rotor to secure the blade to the rotor.

As shown in FIGS. 1 and 2, a shroud 20 is formed at the tip of the airfoil 12 and extends outwardly from the airfoil. The shroud thus has radially inward and radially outward facing surfaces and is exposed to the hot compressed gas flowing through the turbine section. Each shroud has bearing surfaces 22,24 over which it contacts a shroud of an adjacent blade thereby restraining blade vibration. Furthermore, one or more baffles 26 typically extend radially outward from the shroud to prevent leakage of hot gas around the respective blade row. In some conventional bucket blade structures, a plurality of cooling air passages extend radially outwardly through the blade into the blade tip. In other conventional bucket blade structures serpentine passages are defined in the airfoil. As shown in FIG. 2, the radial cooling air passages, conventionally terminate at air discharge holes 28 that allow the cooling air to discharge at the radially outward surface of the shroud.

Directionally solidified (DS) turbine blades/buckets (FIG. 1) are desired in some applications because of the superior mechanical properties exhibited by DS grain structure compared to equiaxed grain structures. Typically, DS grains will grow in the desired direction normal to the chill plate and parallel to the withdrawal direction, but only up to a point. The DS grains will not grow around 90 degree angles or corners. Beyond that, e.g., through radiuses where platforms or tip shrouds connect with airfoils, the grain structure in those surfaces which are more or less perpendicular to the airfoil and the grain growth direction usually end up being equiaxed or something like it. Thus, the grain structure in regions of the turbine blades/buckets such as portions of the tip shroud will not have the desired or required mechanical properties. This invention provides a process whereby a DS grain structure can be obtained from the airfoil through the tip shroud of certain blades/buckets in addition to increasing the quantity of grains in the root of the part.

Before DS and Single Crystal (SC) casting techniques became productionized, investment cast turbine blades and buckets were cast with an equiaxed grain structure. Because of the geometry of these components; i.e. heavier cross-sections in the root and shank ends, tapering to thinner cross-sections at the tip end, gates to allow the molten metal to enter and fill the molds were placed on the heavier ends. These components are usually cast in a tip down attitude in part to take advantage of gravity in the filling and feeding processes required to produce sound castings. The Tip-Down attitude allowed a natural filling and feeding to take place where the metal remained molten longest at the gated end and remained available to feed the casting as it shrank volumetrically on cooling. These structures have different mechanical properties in different crystallographic directions.

The Bridgeman process enabled investment castings to be produced with a controlled crystallographic orientation, so that the superior properties of a specific crystallographic orientation could be utilized. In the DS process, grains nucleate on a chill plate and their growth is controlled by the direction and method of heat extraction. The grains grow normal to the chill plate. They can grow at an angle, but generally they will not grow around corners, i.e., they usually will stop growing as they become parallel to the chill plate (or perpendicular to the withdrawal direction). The grains will also stop growing in the desired direction if/when their growth is interrupted by a surface of the mold that intersects the growth direction (e.g., a tip shroud or a platform).

Since conventional DS castings are produced in the Tip-Down attitude, grain growth begins at the outermost surface of the tip shroud of the bucket. As the grains grow towards the airfoil, those grains that enter the tip shroud from the chill plate (outside of the casting), encounter the airfoil gas path surface of the part (the mold material) and are stopped from growing further. These grains are truncated DS grains, and have the appearance of equiaxed grains when the part is etched. They are not really equiaxed, but are short sections of DS grains, and the properties in these areas are most likely comparable to the transverse properties of the DS grain structure.

The number of grains in the entire structure is limited by the smallest cross-section through which the grains are permitted to grow. Referring to FIG. 3, in the typical casting process of a turbine bucket or aircraft engine blade, as grain growth progresses from the tip shroud 120, a smaller cross-section through the ever increasing cross-section of the airfoil 112, and then through the platform and the still larger cross-section of the shank and the dovetail, shown generally at 114, the number of grains that grew through the small tip shroud 120 does not increase, and in fact may decrease as the larger, faster-growing grains absorb and crowd out the smaller-growing grains. Thus, the limited number of grains present is required to fill the increased cross-section and volume of the part and the dovetail will therefore have fewer grains than the tip shroud. There will be many grains throughout the airfoil that do not extend to or through the dovetail. These grains will be held together only by the strength that bonds them to the adjacent grain. It is desirable to have as many grains as possible held at the dovetail, so that the strength of the component is that of the crystallographic grains, rather than the grain boundary strength.

Thus, in the prior art utilizing the Bridgeman process, the components are aligned with the tip shroud end 120 of the component attached to the heat removal feature 130 (which may for example be a chill plate) as shown in FIG. 3. More specifically, molten metal 132 is poured into a pour cup 134 and then through an appropriate plumbing system which feeds the molten metal ultimately into the part 110. A heat removal feature, for example a chill plate, 130 is disposed to extend horizontally below the mold and is provided as a grain nucleation starter. In the illustrated example the plumbing system is comprised of a vertically oriented sprue 136 which is simply a hollow tube, and a feeder tube 138 that extends from the sprue 136 to the mold.

As illustrated in FIG. 3, the part mold is configured so that the tip shroud 120 is disposed adjacent the heat removal feature, the airfoil 112 extends vertically from the tip shroud to the shank/dovetail mold section 114, and grain growth is in the direction towards the shank/dovetail. These parts are cast in vacuum; i.e. there is no air present and no need for air vents.

An objective of the invention is to create a grain structure in a bucket tip shroud that is more desirable than the result of the prior art. For example, the invention will allow the bucket grain to grow around the tip shroud fillet, producing superior mechanical properties in this highly stressed region of the part. Given the superior properties afforded by the invention, the turbine bucket can be designed to operate at a higher temperature or for a longer duration. In the example of the tip shroud fillet, this invention may eliminate the need to scallop the tip shroud, resulting in superior engine performance. Another objective of the invention is to increase the number of grains in the dovetail (where the part is affixed to the turbine wheel). In the prior art process, the number of grains that extend through the part from the tip shroud 120 to the dovetail 114 is limited by the number of grains that can grow through the airfoil 112 and the footprint of that airfoil where it is attached to the platform/shank. This small footprint, or window, restricted the number of grains of the desired orientation and properties that will reach into the dovetail.

The invention orients the mold for the part 210 such that the root 214 is down so that solidification and grain initiation and growth begins at the opposite end of the component as compared to the prior art and the solidification front moves from the dovetail 214 towards the tip shroud 220; approximately 180 degrees opposite from the solidification and grain growth pattern of the prior art. Thus, referring to the example embodiment depicted in FIG. 4, as in the conventional molding process, molten metal 232 is poured into a pour cup 234 before flowing through the plumbing system into the part 210. A heat removal feature, e.g. a chill plate, 230 is disposed to extend horizontally below the mold as a grain nucleation starter. In the illustrated example embodiment the plumbing system is comprised of a vertically oriented sprue 236 which is simply a hollow tube, and a feeder tube 238 that extends from the sprue 236 to the mold. As an alternative, alloy may be introduced into the part cavity through a non-vertical sprue, and the feeder tube may be disposed in another location, either near the heat removal feature 230 or higher along the part mold.

In contrast to the conventional process described above with reference to FIG. 3, In accordance with embodiments of the invention, the mold is configured so that the shank/dovetail mold section 214 is disposed adjacent the heat removal feature 230, the airfoil 212 extends vertically upwardly from the shank/dovetail to the tip shroud 220, and grain growth is in the direction towards the tip shroud 220.

In accordance with the process of the invention, the larger area of contact with the heat removal feature 230 initiates a larger number of properly oriented grains that then grow through the part from the lager cross-section into the diminishing cross-section. It is desirable to have more correctly aligned DS grains with their superior properties in the highly stressed dovetail region 214. The same grains that will be initiated and held in the dovetail will extend through the length of the airfoil 212. An increase in the quantity of through-going grains will result in an increase in the stress-carrying capability in the component.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. An investment casting process for forming a directionally solidified blade comprising: providing a blade mold, said mold being oriented so that a portion of a mold cavity thereof for forming a base of said blade is at a base thereof and a portion of the mold cavity for forming a tip of the blade is at a vertically upper end thereof; providing a heat removal feature to extend below said mold; flowing molten metal into said mold cavity of said mold; and allowing said blade to solidify from the base upwardly.
 2. The investment casting process of claim 1, wherein said molton metal is fed to a base of said mold cavity.
 3. The investment casting process of claim 1, further comprising providing a sprue adjacent to said blade mold and providing a feeder tube for flowing molten metal from said sprue to said mold cavity.
 4. The investment casting process of claim 3, wherein said molten metal flows through said feeder tube from said sprue into the base of said mold cavity.
 5. The investment casting process of claim 3, wherein said sprue is vertically oriented, adjacent and parallel to said blade mold.
 6. The investment casting process of claim 5, wherein said molten metal flows through said feeder tube from said sprue into the base of said mold cavity.
 7. The investment casting process of claim 5, wherein said flowing molten metal comprises flowing molten metal into the vertically upper end of the sprue before said flowing through said feeder tube.
 8. The investment casting process of claim 1, wherein the mold is configured for forming a turbine bucket, the base of said mold cavity is configured to form a dovetail/shank of the turbine bucket and the vertically upper end of said mold cavity is configured to form a tip shroud of the turbine bucket.
 9. An investment casting assembly for molding a directionally solidified blade comprising: a blade mold, said mold being oriented so that a portion of a mold cavity thereof for forming a base of said blade is at a base thereof and a portion of the mold cavity for forming a tip of the blade is at a vertically upper end thereof; a heat removal feature disposed to extend below said mold; and a plumbing system for flowing molten metal into said mold cavity of said mold, whereby the molded blade will solidify from the base upwardly.
 10. The investment casting assembly of claim 9, wherein said plumbing system comprises a sprue adjacent to said blade mold and a feeder tube for flowing molten metal from said sprue to said mold cavity.
 11. The investment casting assembly of claim 10, wherein said feeder tube extends from said sprue to the base of said blade mold.
 12. The investment casting assembly of claim 10, wherein said sprue is substantially vertically oriented, adjacent and parallel to said blade mold.
 13. The investment casting assembly of claim 12, wherein said feeder tube extends from said sprue to the base of said blade mold.
 14. The investment casting assembly of claim 12, further comprising a pour cup at the vertically upper end of the sprue for receiving molten metal.
 15. The investment casting assembly of claim 9, wherein the mold is configured for forming a turbine bucket, the base of said mold cavity is configured to form a dovetail/shank of the turbine bucket and the vertically upper end of said mold cavity is configured to form a tip shroud of the turbine bucket. 